The present invention relates to acoustic systems for measuring the flow speed of a body of water. More particularly, this invention relates to acoustic systems for measuring the flow speed of a body of water by using at least one sound generator which transmits a sonic pulse through the water to a receiver.
It is often necessary to measure the speed of flow or the speed of the current of a body of water over a period of time and at various points along the path of the current. This information is especially useful in evaluating the effects of hydraulic engineering projects.
Thus, German Patent No. 1,076,980 discloses a method of arranging a measuring sound generator and receiver in a flowing medium. The sound generator emits a pulse which is transmitted over a measuring path to the receiver. The receiver is connected by a signal cable to the measuring sound generator. Upon reception of the pulse transmitted from the sound generator, the receiver transmits a response pulse via the signal cable back to the measuring sound generator. The response pulse triggers the sound generator to emit a subsequent pulse. In this patent, the pulse repetition frequency of the sound generator is proportional to the speed of flow of the water. The sound generator and receiver are enclosed in a streamlined housing in order to minimize errors in the readings due to disturbance of the current by the presence of the generator and the receiver. In order to avoid damage by boat and ship traffic, the sound generator and receiver located at opposite sides of the current to be measured are preferably disposed on the banks of the body of water.
A primary disadvantage of this system is that it requires the use of a signal cable, which must extend across the body of water to connect the sound generator to the receiver so that the receiver can transmit a trigger pulse back to the sound generator. Accordingly, such flow measurements have previously been made only in the vicinity of a bridge over which the signal cable could be laid. It is possible, if no bridge is available, to lay the signal cable across the body of water, but the cable must then be buried in the bed of the body of water because a cable placed on top of the bed will soon be damaged by moving sedimentary material borne by the current of the body of water. Moreover, it would be necessary to prohibit anchoring at the location of the cable. The expense of overcoming the problems of laying the signal cable in the bed of the body of water is thus prohibitive.
German Auslegeschrift [Published Application] No. 1,648,147, discloses an arrangement for measuring the flow of liquid in a pipe having parallel walls wherein the measuring sound generator and the receiver are both attached to the same wall of the pipe. With this arrangement the reflections of a pulse from the opposite wall of the pipe are utilized to return the pulse emitted by the measuring sound generated to the receiver. Such an arrangement, however, cannot be used with bodies of water such as rivers having natural shore lines which cannot produce a satisfactory pulse reflection.
In view of the foregoing considerations there is a need for a system which can measure the speed of flow or of the current of a body of water in any desired location along, or in, the body of water without using a cable to connect the measuring sound generator and receiver.